Cement, concrete or mortar is widely used as an inexpensive construction material. However, these materials are damaged by an acid, salt, seawater, and an oil or under chemical environments such as carbonic acid gas and sulfurous acid gas. In order to compensate for this drawback, the surface of concrete is conventionally coated with a corrosive-resistant paint. This is called "coating" when a thin film having a thickness of 0.2 to 0.3 mm is formed and is called "lining" when a thick film having a thickness of 1 mm or more is formed. However, this distinction is not always clear.
Asphalt, tar, or a thermosetting resin paint is normally used in coating. In corrosion-resistant coating of a concrete structure, it is most important to cover the surface of concrete so that no pin-holes are left thereon. However, in practice, it is not easy to coat the surface of hardened concrete in such manner. That is, since a large number of large and small holes are present on the concrete surface, it is very difficult to coat the surface so that all the holes including pin-holes are completely covered. For this reason, in conventional techniques, the number of coating layers is increased or a primer treatment is performed to solve the above problem. However, these techniques require much labor. For example, in "Guides (Draft) for Salt Damage of Road Bridge and its Explanation" (Incorporated Body (Shadan Hojin) of Japan Road Association, issued on February, 1984), concrete painting systems for preventing salt damage are classified in three types of A, B, and C in accordance with using conditions of a structural member. In any of these painting systems, four steps of a primer treatment (first treatment) using an epoxy resin primer or a polyurethane resin primer, a putty treatment (second treatment) using an epoxy resin putty or a vinyl ester resin putty, intercoating (third treatment) using an epoxy resin paint, a polyurethane resin paint, or a vinyl ester resin paint, and over-coating (fourth treatment) using a polyurethane resin are required. In addition, it is standardized that an interval of one to ten days be provided between the steps. In order to improve adhesive properties between the film and the concrete to sufficiently achieve a corrosive preventing effect, materials which adversely affect painting such as a laitance, a mold releasing agent, attached salt, attached oil, and moisture must be removed. As a base treatment for this purpose, the surface is sanded, wiped with a thinner, cleaned by a brush, or dried by air blow.
However, these treatments are very troublesome and costly. In addition, a film formed even upon these treatments is easily peeled. Other examples of corrosion-resistant concrete are polymer-impregnated concrete and resin concrete. However, these examples are costly and hence are not suitable for use in general purposes.
According to the present invention, the inner surface of a formwork is coated beforehand to form the corrosion-resistant film thereon, and concrete or mortar is molded using this formwork. Then, a corrosion-resistant film layer is transferred onto the surface of the concrete or mortar at the same time the concrete or mortar is removed from the formwork. As a result, concrete or mortar having a good acid resistance can be easily obtained.